Blood pressure measurement apparatus

ABSTRACT

Disclosed is a blood pressure measurement apparatus in which a waveform discrimination method is used in the recognition of Korotkoff sounds in the measurement of blood pressure by auscultation. In processing executed by the apparatus, the minimum point or maximum point (C3) of a signal waveform indicative of the sound of vibration produced by a blood vessel is detected, and a maximum value or minimum value point (C2) is detected within a predetermined time region (t 1 ) the final instant of which is the point (C3). If a level difference between the point (C2) and the point (C3) is within a predetermined range, there is detected a minimum value or maximum value point (C1) within a predetermined time region (t 2 ) the final instant of which is (C2). If a level difference between the point (C1) and the point (C2) is within a predetermined range, there is detected a maximum value or minimum value point (C4) within a predetermined time region (t 3 ) the starting instant of which is the point (C3). It is then discriminated whether a level difference between the point (C4) and the point (C3) falls within a predetermined range. Control proceeds in successive fashion when each of these conditions is satisfied, with a Korotkoff sound being recognized in the signal waveform when all of these conditions are satisfied.

BACKGROUND OF THE INVENTION

(1) Field of the Invention:

This invention relates to a blood pressure measurement apparatus andmethod in which a waveform discrimination method is used in therecognition of Korotkoff sounds in the measurement of blood pressure byauscultation.

(2) Description of the Prior Art:

In the detection of Korotkoff sounds according to the prior art, themost widespread approach is a discrimination method using a filter andcomparator. This is referred to as the filter comparator method. Anotherapproach used much less widely is a discrimination method, namely apattern recognition method, which is based on the waveform of theKorotkoff sounds.

It is known that the spectral distribution of Korotkoff sounds generallyhas a frequency component different from body movement and externalnoise. The filter comparator method utilizes this fact and measuresblood pressure by filtering a signal detected by a microphone attachedto a pressure cuff fastened to a patient's arm, reducing the amplitudeof frequency components other than the frequency component of theKorotkoff sounds, then comparing the frequency component of theKorotkoff sounds with a preset threshold value by means of a voltagecomparator, and discriminating this frequency component based on itsmagnitude.

However, the frequency component of Korotkoff sounds not only variesfrom one patient to another but also differs for one and the samepatient depending upon such measurement conditions as the time at whichmeasurement is made and cuff pressure. Moreover, since the frequencyband of interest is fairly wide, ranging from several tens of Hertz to200-300 Hz, it is very difficult to extract solely the Korotkoff soundcomponent by removing the sound of the patient's pulse and noise.

When the frequency component of the Korotkoff sounds is small incomparison with the sound of the patient's pulse, it is difficult todistinguish between the pulse sound and the Korotkoff sounds.Furthermore, since the discrimination is made based on a voltage level,measurement precision is readily influenced by any disparity in theamplitude of the Korotkoff sounds.

The aforementioned pattern recognition method in which discrimination ismade based on the waveform of the Korotkoff sounds has recently been putinto partial practical use.

In general, the waveform of a Korotkoff sound is as shown in FIG. 2(A).The waveform is subjected to an A-D conversion so as to make it easierto process the sound data detected by a pick up, with the digital signalresulting from the conversion being stored in means such as a memory.This is referred to as pattern detection processing. Next, maximum andminimum values are calculated from the stored signal values. Forexample, characteristic points are successively detected, as shown atC1, C2, C3, C4 in FIG. 3(A), four of such points being the minimumnumber necessary. This is referred to as a characteristic point plottingstep. After the characteristic points have been detected, the generalposition of each characteristic point is verified and a decision isrendered as to whether the waveform is indeed indicative of a Korotkoffsound. This step is referred to as a discrimination processing step.Thus, recognition processing is divided into three process blocks.

If a characteristic point is not detected in the characteristic pointplotting processing step, the pattern detection processing step isreturned to for further signal read in. If a decision is rendered in thediscrimination step to the effect that the waveform is not that of aKorotkoff sound, processing is executed for detecting furthercharacteristic points or for reading in a new signal.

The relationship among the pattern recognition processing blocks isillustrated in FIG. 6.

A problem encountered in the pattern recognition approach is that in theactual measurement data obtained from a living body, a fine ripple shownin FIG. 7 tends to be produced in the vicinity of the maximum andminimum points of the Korotkoff sound signal owing to the influence anA-D conversion error.

Accordingly, with the method of detecting maximum and minimum values oneby one while traversing the signal waveform in regular order and thentreating each such value as a characteristic point, there is very largeamount of feedback from the discrimination processing and, hence, themethod requires a considerable period of time for execution. Inaddition, there is strong possibility that characteristic points will bedetected erroneously.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a bloodpressure measurement apparatus and method adapted to detect a maximumvalue or minimum value of an extreme point capable of being traversed bya Korotkoff sound signal constituent within a predetermined time regionin which detected characteristic points of detected blood vesselinformation serve as a reference, perform a Korotkoff sound recognitionby comparing characteristic points, and recognize Korotkoff soundsaccurately without the influence of a ripple component produced in thevicinity of the extreme values of the Korotkoff sounds.

Another object of the present invention is to provide a blood pressuremeasurement apparatus and method in which a Korotkoff sound recognitionis performed with detected characteristic points serving as a reference,whereby extreme value points having the greatest certainty of beingdetected in a Korotkoff signal waveform can be treated as referencecharacteristic points.

According to the present invention, the foregoing objects are attainedby providing a blood pressure measurement apparatus comprising: bloodvessel information detecting means for detecting a signal waveform of asound or vibration produced by a blood vessel; holding means for holdingthe signal waveform detected by the blood vessel information detectingmeans; maximum point detecting means for detecting a maximum point (C3)of the waveform held by the holding means; first C-point detecting meansfor detecting a minimum value point (C2) within a predetermined timeregion (t₁) the final instant of which is the maximum point (C3)detected by the maximum point detecting means; first discriminatingmeans for discriminating whether a level difference between the minimumvalue point (C2) detected by the first C-point detecting means and themaximum point (C3) falls within a predetermined range; second C-pointdetecting means for detecting a maximum value point (C1) within apredetermined time region (t₂) the final instant of which is thedetected minimum value point (C2), this being performed when the firstdiscriminating means discriminates that the level difference fallswithin the predetermined range; second discriminating means fordiscriminating whether a level difference between the maximum valuepoint (C1) detected by the second C-point detecting means and theminimum value point (C2) falls within a predetermined range; thirdC-point detecting means for detecting a minimum value point (C4) withina predetermined time region (t₃) the starting instant of which is themaximum point (C3), this being performed when the second discriminatingmeans discriminates that the level difference falls within thepredetermined range; third discriminating means for discriminatingwhether a level difference between the minimum value point (C4) detectedby the third C-point detecting means and the maximum point (C3) fallswithin a predetermined range; and a control unit for starting at leastthe three C-point detecting means and three discriminating means,advancing control successively when the respective conditions arerealized, and recognizing a Korotkoff sound in the signal waveform whenthe third discriminating means discriminates that the level differencebetween the minimum value point (C4) and maximum point (C3) falls withinthe predetermined range.

According to a preferred embodiment of the present invention, thecontrol unit includes control means for restarting control fromdetection of the maximum point (C3) when the discrimination conditionfor any of the level discriminating means fails to be satisfied, and theblood vessel sound detecting means includes setting means for setting athreshold value of a detection signal in accordance with the magnitudeof a Korotkoff sound recognized immediately before.

Further, the control unit includes holding means for holding the signalwaveform at every predetermined time.

The foregoing objects may also be attained by providing a blood pressuremeasurement apparatus comprising: blood vessel information detectingmeans for detecting a signal waveform of a sound or vibration producedby a blood vessel; holding means for holding the signal waveformdetected by the blood vessel information detecting means; minimum pointdetecting means for detecting a minimum point (C3) of the waveform heldby the holding means; first C-point detecting means for detecting amaximum value point (C2) within a predetermined time region (t₁) thefinal instant of which is the minimum point (C3) detected by the minimumpoint detecting means; first discriminating means for discriminatingwhether a level difference between the maximum value point (C2) detectedby the first C-point detecting means and the minimum point (C3) fallswithin a predetermined range; second C-point detecting means fordetecting a minimum value point (C1) within a predetermined time region(t₂) the final instant of which is the detected maximum value point(C2), this being performed when the first discriminating meansdiscriminates that the level difference falls within the predeterminedrange; second discriminating means for discriminating whether a leveldifference between the minimum value point (C1) detected by the secondC-point detecting means and the maximum value point (C2) falls within apredetermined range; third C-point detecting means for detecting amaximum value point (C4) within a predetermined time region (t₃) thestarting instant of which is the minimum point (C3), this beingperformed when the second discriminating means discriminates that thelevel difference falls within the predetermined range; thirddiscriminating means for discriminating whether a level differencebetween the maximum value point (C4) detected by the third C-pointdetecting means and the minimum point (C3) falls within a predeterminedrange; and a control unit for starting at least the three C-pointdetecting means and three discriminating means, advancing controlsuccessively when the respective conditions are realized, andrecognizing a Korotkoff sound in the signal waveform when the thirddiscriminating means discriminates that the level difference between themaximum value point (C4) and minimum point (C3) falls within thepredetermined range.

The control unit includes control means for restarting control fromdetection of the minimum point (C3) when the discrimination conditionfor any of the level discriminating means fails to be satisfied.

The blood vessel sound detecting means includes setting means forsetting a threshold value of a detection signal in accordance with themagnitude of a Korotkoff sound recognized immediately before.

The apparatus further includes inverting means for inverting, withrespect to a reference level, a signal waveform held by detection of theminimum point by the minimum point detecting means, wherein a value ofan output inverted by the inverting means is used as a reference formaximum/minimum value detection and level discrimination performed by atleast the three C-point detecting means and three discriminating means.

The control unit includes holding means for holding the signal waveformat every predetermined time.

Further, according to the present invention, there is provided a bloodpressure measurement method which comprises: a blood vessel informationdetecting step for detecting a signal waveform of a sound or vibrationproduced by a blood vessel; a holding step for holding the signalwaveform detected at the blood vessel information detecting step; amaximum point detecting step for detecting a maximum point (C3) of thewaveform held at the holding step; a first C-point detecting step fordetecting a minimum value point (C2) within a predetermined time region(t₁) the final instant of which is the maximum point (C3) detected atthe maximum point detecting step; a first discriminating step fordiscriminating whether a level difference between the minimum valuepoint (C2) detected at the first C-point detecting step and the maximumpoint (C3) falls within a predetermined range; second C-point detectingmeans for detecting a maximum value point (C1) within a predeterminedtime region (t₂) the final instant of which is the detected minimumvalue point (C2), this being performed when it is discriminated at thefirst discriminating step that the level difference falls within thepredetermined range; a second discriminating step for discriminatingwhether a level difference between the maximum value point (C1) detectedat the second C-point detecting step and the minimum value point (C2)falls within a predetermined range; third C-point detecting step fordetecting a minimum value point (C4) within a predetermined time region(t₃) the starting instant of which is the maximum point (C3), this beingperformed when it is discriminated at the second discriminating stepthat the level difference falls within the predetermined range; a thirddiscriminating step for discriminating whether a level differencebetween the minimum value point (C4) detected at the third C-pointdetecting step and the maximum point (C3) falls within a predeterminedrange; and a step of starting at least the three C-point detecting stepsand three discriminating steps, advancing control successively when therespective conditions are realized, and recognizing a Korotkoff sound inthe signal waveform when it is discriminated at the third discriminatingstep that the level difference between the minimum value point (C4) andmaximum point (C3) falls within the predetermined range.

The present invention further provides a blood pressure measurementmethod which comprises: a blood vessel information detecting step fordetecting a signal waveform of a sound or vibration produced by a bloodvessel; a holding step for holding the signal waveform detected at theblood vessel information detecting step; a minimum point detecting stepfor detecting a minimum point (C3) of the waveform held at the holdingstep; a first C-point detecting step for detecting a maximum value point(C2) within a predetermined time region (t₁) the final instant of whichis the minimum point (C3) detected at the minimum point detecting step;a first discriminating step for discriminating whether a leveldifference between the maximum value point (C2) detected at the firstC-point detecting step and the minimum point (C3) falls within apredetermined range; second C-point detecting means for detecting aminimum value point (C1) within a predetermined time region (t₂) thefinal instant of which is the detected maximum value point (C2), thisbeing performed when it is discriminated at the first discriminatingstep that the level difference falls within the predetermined range; asecond discriminating step for discriminating whether a level differencebetween the minimum value point (C1) detected at the second C-pointdetecting step and the maximum value point (C2) falls within apredetermined range; a third C-point detecting step for detecting amaximum value point (C4) within a predetermined time region (t₃) thestarting instant of which is the minimum point (C3), this beingperformed when it is discriminated at the second discriminating stepthat the level difference falls within the predetermined range; a thirddiscriminating step for discriminating whether a level differencebetween the maximum value point (C4) detected at the third C-pointdetecting step and the minimum point (C3) falls within a predeterminedrange; and a step of starting at least the three C-point detecting stepsand three discriminating steps, advancing control successively when therespective conditions are realized, and recognizing a Korotkoff sound inthe signal waveform when it is discriminated at the third discriminatingstep that the level difference between the maximum value point (C4) andminimum point (C3) falls within the predetermined range.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the basic construction of aKorotkoff sound recognition apparatus embodying the present invention;

FIGS. 2(A) and 2(B) are views showing typical patterns of Korotkoffsounds;

FIGS. 3(A) and 3(B) are views showing characteristic portions of aKorotkoff sound waveform;

FIG. 4(A) and 4(B) is a flowchart illustrating processing extending fromdetection of each characteristic point of a Korotkoff sound torecognition of a Korotkoff sound according to an embodiment of thepresent invention;

FIGS. 5(A) to 5(O) are views illustrating the recognition states of eachcharacteristic point of a Korotkoff sound waveform when the processingindicated by the flowchart of FIG. 4 is executed;

FIG. 6 is a block diagram illustrating a conventional Korotkoff sounddiscrimination method based on waveform configuration; and

FIG. 7 is a view showing a Korotkoff signal waveform in which a smallripple is produced in the vicinity of extreme values.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will now be described in detailwith reference to the drawings.

FIG. 1 is a block diagram illustrating the basic construction of anembodiment of the present invention. The arrangement includes amicrophone 1 for picking up Korotkoff sounds (hereafter referred to as"K-sounds") and for producing an analog output signal indicativethereof. The input analog signal between range of 0.3 V and 2.0 V isconverted into a 8 bit digital signal every 4 milliseconds (ms) by ananalog-digital (A/D) converter 2 before being applied to an arithmeticcircuit 3. The latter serves as recognition means and is adapted torecognize a K-sound by processing a series of sound data signalsobtained in digital form from the A/D converter 2. The arithmeticcircuit 3 comprises a one-chip CPU having a RAM and a ROM and is soillustrated that the various functions implemented by executing aprogram stored in the ROM are shown in block form. The present inventionis capable of implementing these functions efficiently with the limitedmemory and limited processing time given the CPU. Numeral 4 denotes adisplay unit for displaying the fact that a K-sound has been recognized,for displaying other information as well.

The arithmetic circuit 3 includes a data read-in unit 5 for reading thedigital output signal from the A/D converter 2 into the arithmeticcircuit 3, a threshold value setting unit 6 which compares the newlyread digital signal data from the data read-in unit 5 and the thresholdvalue determined based upon the most recent K-sound. The arithmeticcircuit 3 further includes a time generator for generating timeinformation, and a memory (RAM) 9 for storing a digital data as well asthe time information prevailing at the instant of detection. Thethreshold value which is to be held in the threshold value setting unit6 is calculated from the most recent K-sound in a K-sound recognitionunit 15 in accordance with the following equation,

    |C3-P.sub.o |·β1,          (1)

where

C3: the characteristic point of the detected K-sound

P_(o) : reference level

β1: one-third (1/3)

If the newly read digital signal is larger than the most recentthreshold value, the sign bit is set to "1". This sign bit is includedin the output 106. If, on the contrary, the new data is smaller than thethreshold value, the sign bit is set to "0". After completion of thisoperation, the digital data signal containing the sign bit is deliveredto the memory 9 as digital signal data 106. The sign bit is referred toby a C3 detector 10 in order to determine whether to produce a signal online 113 or not.

Before continuing with the description of the functional blocks of theapparatus shown in FIG. 1, let us discuss typical patterns of theK-sounds which are to be recognized by the apparatus.

FIG. 2(A) is a typical pattern of a K-sound waveform recognized by theapparatus of the illustrated embodiment, and FIG. 2(B) illustrates thepattern when the signal level is inverted. As it is possible that theinput waveform may represent two opposite waveforms, drawingsillustrating the respective waveforms, for example, FIGS. 2(A) and 2(B),are provided in this application. The characteristic points of theK-sound waveform are the four points C₁ -C₄ shown in FIGS. 3(A), (B). Inorder to help understanding of the present invention, level differencesdP1 to dP3 and time regions T₁ to T₃ are labeled in FIG. 3A. They willbe referred to in the following discussion. In the illustratedembodiment, a K-sound is recognized on the basis of the relationshipamong these four points.

In FIGS. 3(A), (B), the point C3 is defined as the point where thesignal level attains an absolute extreme value, i.e., the highest peakor lowest valley, and is a portion which has great significance for thepurpose of K-sound recognition, described below. Specifically, once thecharacteristic point C3 has been found, the characteristic points C1,C2, C4 are each obtained by a prescribed analytical method which startsfrom the point C3.

Each of the functional blocks described below constitutes means forrecognizing the abovementioned K-sound waveform patterns both reliablyand efficiently.

Returning to FIG. 1, numeral 10 is the C3 detector for detecting arelative (local) extreme value, i.e., a maximum value or minimum valuein the digital signal data read out of the memory 9. Numeral 11designates a level inverter which, for the purpose of K-soundrecognition, inverts the level of the signal waveform data, which isread out of the memory 9, whenever necessary. A characteristic pointdetector 12 performs a predetermined calculation with regard to thesignal waveform data read out of the memory 9 and time data to check forthe presence of a waveform located at each of the characteristic pointsC1, C2, C4. The characteristic point detector 12 comprises a time regionsetting unit 13 for generating prescribed time region data, and aK-sound discriminator 14 for discriminating whether sound data of asignal level forming a characteristic is present within the time region,and is adapted to detect each characteristic point in accordance with apredetermined calculation procedure, described below, when a signalindicating that the C3 point has been detected is received from the C3detector 10. The output of the K-sound discriminator 14 is applied to aK-sound recognition unit 15, which examines the positional relationshipamong a collection of characteristic points found by the characteristicpoint detector 12, in order to recognize a K-sound.

The operation of the present embodiment comprising the foregoingelements will now be described.

The output of the microphone 1 is an analog electric signal 101indicative of a K-sound picked up by the microphone. The signal 101 isconverted into a digital signal 102 at every 4 ms sampling instant bythe A/D converter 2. The digital signal 102 at the output of the A/Dconverter 2 is read into the arithmetic circuit 3 by the data read-inunit 5 and is applied to the threshold value setting unit 6 as a seriesof digital signal data 103 in a time series. The threshold value settingunit 6 sets a threshold value in dependence upon a signal 117 from theK-sound recognition unit 15 indicative of the magnitude of a K-soundwhich appeared last in accordance with equation 1. By setting thethreshold value to |C3-P_(o) |·β1, the unit 6 reduces the influence ofnoise contained in the waveform data 103.

In order to suitably deal with a signal pattern input of any amplitudewhatsoever at the start of measurement, no threshold value is set whenmeasurement starts. After measurement starts, however, a threshold valueis set upon predicting, from the magnitude of an immediately precedingK-sound, the smallest magnitude capable of being traversed by the nextK-sound. More specifically, in a case where a K-sound has alreadyappeared in the course of measurement, the threshold value setting unit6 sets a threshold value in dynamic fashion in dependence upon thesignal 117 from the K-sound discriminator 15 indicative of the magnitudeof the threshold |C3-P_(o) |·β1, thereby making it possible to detectthe C3 point accurately and rapidly.

Accordingly, the threshold value setting unit 6 in the apparatus of theillustrated embodiment is different in nature from threshold valuesetting means in the conventional comparator method, in which athreshold value is set that is fixed with respect to the amplitude ofthe K-sound. The threshold value setting unit 6 delivers the digitalsignal data 106 to the memory 9 each 4 ms interval and also deliverstiming signal 105 to the time generator 8 each sampling instant at theA/D converter 2.

The time generator 8 is a unit which cyclically counts timinginformation that increases every millisecond, by way of example. Whenthe timing signal 105 is received from the threshold value setting unit6, the time generator 8 successively counts up a write-in address 120 ofthe memory 9 so that the digital signal data 106 from the thresholdvalue setting unit 6 and prevailing clocked time information 107 arewritten into the memory in accordance with the counted up address. Thus,the digital signal value data 106 and the time information 107prevailing at the moment of detection are stored in the memory 9.

The time generator 8 also outputs a read-out address 120 of the memory 9at a predetermined time interval and produces a read enable signal 121when a read-out becomes possible.

The digital signal data 106 stored in memory 9 is read by the C3detector 10 and characteristic point detector 12 in accordance with theread enable signal 121, whereby K-sound recognition processing isperformed. In order that the memory 9 can be read from any address whenK-sound recognition processing is executed, the characteristic pointdetector 12 provides the time generator 8 with an address designatingsignal 122 for designating a read-out address from which a read-out isto be started.

The K-sound recognition processing will now be described with referenceto the flowchart of FIGS. 4(A) and 4(B).

In accordance with the read enable signal 121 from the time generator 8,the C3 detector 10 reads sound data 108 out of the memory 9 inaccordance with the successively stored time series, examines these datain regular order and executes processing for detecting the C3 point inthe signal pattern shown in FIG. 3(A) or FIG. 3(B).

In the first step S90 of the flowchart shown in FIGS. 4(A) and (B), aninversion flag 10a internally of the C3 detector 10 is set to "0". Whenthe inversion flag 10a is "0", an inversion indicating signal 109 isreset; when the flag 10a is "1", the inversion designating flag 109 isset. When the inversion indicating signal 109 is in the reset state, thelevel inverter 11 delivers read-out data 110 from memory 9 directly tothe characteristic point detector 12 as output data 111. When theinversion indicating signal 109 is in the set state, the level inverter11 inverts the read-out data from the memory 9 and delivers the resultto the characteristic point detector 12 as the output data 111.

Initially, the inversion flag 10a is set to "0" and the inversiondesignating signal 109 is reset. Consequently, the read-out data frommemory 9 is applied as such to the characteristic point detector 12.

Next, at a step S9l, the C3 detector 10, in accordance with the readenable signal 121, reads the digital signal data 106 from the thresholdvalue setting unit 6 stored successively in memory 9 via, the A/Dconverter 2 out of the memory in the same order in which it was storedand compares this with digital signal data 106 read out immediatelybefore. The time generator 8 exercises read-out control separate fromthe write-in of the digital signal data 106. The reading of data fromthe memory 9 can be performed immediately by writing in the digitalsignal data 106 by means of the threshold value setting unit 6. Extremevalue detection processing is executed from step S92 onward and isperformed by a level comparison of digital signals at three consecutivepoints in the sound data 108.

Thus, at the step S92, digital signals at three consecutive points arecompared to determine whether a valley point has been detected, that is,to check whether the level difference between adjacent ones of thepoints changes from a decreasing value to an increasing value. If avalley point is detected and this valley point is given a sign bit "1",that is the newly obtained digital signal data is determined by the C3detector 10 to be larger than the current threshold value, this point istreated as being the characteristic point C3 and the program proceedsfrom the step S92 to a step S94, at which the inversion flag 10a is setto "1" and the inversion indicating signal 109 is delivered to the levelinverter 11. The program then proceeds to a step S95.

When the inversion indicating signal 109 is delivered to the levelinverter 11, K-sound detection is performed. To this end, the level ofeach item of waveform data 110 corresponding to the characteristicpoints C₁ -C₄ in FIG. 3(B) and read out of the memory 9 is inverted withregard to a base line [level P_(o) shown in FIGS. 5(D), (E)] that willturn these levels into the signal pattern shown in FIG. 3(A). Theinverted levels are delivered to the characteristic point detector 12.

If a valley point is not detected at the step S92, the program proceedsto a step S93, at which it is determined whether a peak C3 has beendetected, that is, whether the level difference between adjacent ones ofthe three consecutive points changes from an increasing value to adecreasing value. If a peak is detected, this peak is determined as towhether the peak is exceeding the threshold value with referring to thesign bit included in the digital signal data 106. If the peak isexceeding the threshold value, the peak is treated as being thecharacteristic point C3 and the program proceeds to a step S95. If apeak is not detected at the step S93, the program returns to the stepS9l, the next item of digital signal data is read and processing fordetecting a characteristic point C3 is performed again.

If a peak point is detected at the step S93, the inversion flag 10aremains at "0" and the program proceeds to the step S95. This step callsfor a characteristic point detection signal 113 to be sent from theC-point detector 10 to the characteristic point detector 12 to indicatethat the detected point is a characteristic point. The program thenproceeds to a step S104.

If a K-sound is recognized, what is detected first is the peak point. Anexample of the state in which the initial peak is detected isillustrated in FIG. 5(A).

In response to the characteristic point detection signal 113 from the C3detector 10 indicating that the C3 point has been detected, thecharacteristic point detector 12 initiates detection of eachcharacteristic point of the digital signal data constituting a K-sound.This being performed by processing from step S104 onward.

When the characteristic point detection signal 113 is received, thecharacteristic point detection circuit 12 produces the addressdesignating signal 122 so that data stored in the memory 9 prior todetection of the characteristic point are read out of the memorysequentially in the same order that the data were stored. These data arestored in a RAM. In other words, each item of data from C1 to C3 isstored every 4 ms sampling period in the RAM at the instant C3 isdetected.

The step S104 calls for the time region setting unit 13 to set apredetermined time region t₁ the final instant of which is the positionof C3. The unit 13 produces a time region signal 114 indicative of thistime region and applies the signal to the K-sound discriminator 14. Thesetting of this time region can be accomplished by storing in a ROM apredetermined value in accordance with the figures given in below. Theset time region t₁ is illustrated in FIG. 5(B). According to theembodiment of the present invention, each time region t₁, t₂ and t₃ havethe values t₁ =10, t₂ =15 and t₃ =15 [×4 ms]. And these time region datahave been stored previously in the ROM which is constituting the timeregion setting unit 13.

Next, the program proceeds to a step S105, at which the K-sounddiscriminator 14 reads digital signal data within the time region t₁ setby the time region setting unit 13 and stored in the RAM, detects aminimum value within the read data and treats this value as C2 [FIG.5(C)]. The minimum level point is detected by comparing the levels oftwo points in the output data 111 from the level inverter 11. Next, astep S106 calls for a decision as to whether the level difference (dP2)between C2 and C3 falls within a predetermined range. The upper andlower limits of this range are stored beforehand in the ROM inaccordance with the below table. According to the embodiment of thepresent invention, each level difference dP1, dP2 and dP3 is given asfollows.

    ______________________________________                                                   lower limit                                                                           upper limit                                                ______________________________________                                        dP2          15        --                                                     dP1          dP2 × α1                                                                    dP2 × α2                                   dP3          dP1 × α3                                                                    dP2 × α4                                   ______________________________________                                    

In the above table, the unit is 0.7 V/256 and also, the leveldifferences illustrated in FIG. 3A, dP1, dP2 and dP3, are given below.

dP1: voltage difference between C1 and C2

dP2: voltage difference between C3 and C2

dP3: voltage difference between C3 and C4

In FIG. 5(C), the level difference is not within the predeterminedrange, so that the program returns to the step S90 for detection of thenext characteristic point.

A valley point shown in FIG. 5(D) is detected by subsequent C3 detectionprocessing. The program proceeds from the step S92 to the step S94, theinversion flag 10a is set and, in the data read out of the memory 9 andstored in the RAM, the input signal level is level-converted from P to2P_(o) -P (where P_(o) is a reference level) by the level inverter 11,thereby giving the waveform shown in FIG. 5(E). Thus, the signalwaveform is apparently inverted. Next, the minimum point C2 is detectedwithin the set time region t₁ through the steps S104, S105 [FIG. 5(F)].The decision step S106 now finds that the point C2 lies within thepredetermined limits, so that the program proceeds from this step to astep S107, where the time region setting unit 13 sets a predeterminedtime region t₂ the final instant whereof is the position of C2. A signal114 indicative of this time region is delivered to the K-sounddiscriminator 14. The set time region t₂ is shown in FIG. 5(G).

Next, the program proceeds to a step S108, at which the point (value) ofa maximum level is detected within the time region t₂ and treated as C1.The detection of the maximum level is performed by comparing the levelbetween two points. This is followed by a step S109, at which it isdetermined whether the level difference dP1 between C1 and C2 lies witha predetermined range. In FIG. 5(H), detection of C1, C2, C3 is judgedto be improper and the program returns to the step S90.

The characteristic point detected next is C3 shown in FIG. 5(I) and noinversion is made of the read waveform by the level inverter 11. At thestep S104, the time region t₁ is set as shown in FIG. 5(J), thecharacteristic point C2 shown in FIG. 5(K) is detected at the step S105,the time region t₂ having the characteristic point C2 as its finalinstant is set at the step S107, as shown in FIG. 5(L), and the maximumvalue C1 is detected within the time region t₂ shown in FIG. 5(M) at thestep S108. This is followed by the level decision step S109, at which itis judged that detection of C1, C2, C3 is proper.

The program then proceeds to a step S110, at which the time regionsetting unit 13 sets a time region t₃ the final instant of which is theposition of C3, as shown in FIG. 5(N). Next, at a step S111, a digitalsignal data 103 within the time region t₃ is read out of the memory 9,the point of a minimum level is detected and the point is treated asbeing C4. This is shown in FIG. 5(O). This is followed by a step S112,at which it is determined whether the level difference dP3 between C3and C4 falls within a predetermined range.

If the level difference does not fall within the predetermined range,the program returns to the step S90. If the level difference does fallwithin the predetermined range, the signal is recognized to be a K-soundat a step S113.

Since the present method is a very simple method of detecting thecharacteristic points C1, C2, C3, C4, it is suited to real-timeprocessing performed by a one-chip CPU. Furthermore, since the maximumvalue of a peak or the minimum value of a valley is detected within eachpredetermined time period, the influence of noise (FIG. 7) produced inthe vicinity of extreme values of the K-sound waveform is almost nil.

When the K-sound discriminator 14 successively detects the positions C3,C2, C1 and C4, an output 115 is produced so as to inform the K-soundrecognition unit 15 that the digital signal data should be treated as aK-sound. When the K-sound is recognized by the unit 15, the unitperforms a computation adapting equation 1 to obtain and to deliver thenew threshold value to the threshold value setting unit 6 in order torenew the current threshold value. The recognized K-sound is deliveredfrom the K-sound recognition unit 15 to the display unit via line 116.

In the illustrated embodiment, an example has been described in whichcharacteristic points are detected upon inverting peaks and valleys ofthe signal waveform as reference characteristic points whenevernecessary. However, it is permissible to execute processing withoutmaking the inversion or to treat a reference characteristic point solelyas the waveform peak. In such case, the inversion flag and levelinverter 11 can be deleted.

ADVANTAGES OF THE INVENTION

According to the present invention as described above, thecharacteristics of a K-sound waveform are investigated directly. As aresult, it is unnecessary to place a restriction upon the frequency bandcharacteristic of a filter or to set a threshold value fixed withrespect to the amplitude of a K-sound, as in the prior art. Moreover,measurement precision is not readily influenced by the frequencycomponent constituting the K-sound or by the effect of a disparity inthe amplitude of the K-sound.

Further, according to the present invention, the maximum value orminimum value capable of being traversed by a K-sound signal constituentis detected within each set time region, unlike the conventional methodin which maximum and minima are detected one by one while traversing thewaveform in regular order. Accordingly, the detection of candidates forcharacteristic points can be readily performed by a very short programand measurement precision is not influenced by fine ripple produced inthe vicinity of extreme values of a K-sound waveform, particularlyripple due to a conversion error which readily occurs after the A/Dconversion.

According to the present invention, a plurality of signal patternsindicative of K-sounds can be recognized efficiently on a real-timebasis in the limited memory and processing time given a one-chip CPU byusing simple software programmed in such a manner that typical patternsof K-sound waveforms are recognized. By adopting an arrangement in whichlevel inverting means is provided, a plurality of patterns can berecognized by a short program.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

What is claimed is:
 1. A blood pressure measurement apparatus,comprising:blood vessel information detecting means for detectingvibration of a blood vessel to produce a signal waveform; storage meansfor storing the signal waveform produced by said blood vesselinformation detecting means and time information associated with thesignal waveform; reference absolute extreme value point locating meansfor operating on the signal waveform stored in said storage means inresponse to a conditional command to locate a reference absolute extremevalue point of the signal waveform stored in said storage means, thereference extreme value point used as a base point for time durationmeasurement, said reference absolute extreme value point locating meanscomprising:first extreme value detecting means for detecting one offirst and next extreme value points in the signal waveform stored insaid storage means; polarity determining means for determining polarityof the one of the first and next extreme value points and for invertinga local portion of the signal waveform when the polarity is negative;and means for providing as an output the reference absolute extremevalue point representative of the one of the first and next extremevalue points detected by said first extreme value detecting means;C-point value searching means, responsive to the time information and tothe output of said reference absolute extreme value point means, forfinding relative extreme value points with respect to the referenceextreme value point in the signal waveform stored in said storage meansto thereby validate existence of a Korotkoff waveform, said C-pointvalue searching means comprising:means for receiving the local portionof the signal waveform stored in said storage means; second extremevalue detecting means for detecting the relative extreme value points inthe local waveform portion in predeterined time intervals referenced tothe reference absolute extreme value point in the signal waveform storedin said storage means, and for providing an occurrence output indicativethereof; and level determining means for determining whether leveldifferences between adjacent ones of the reference absolute and relativeextreme value points fall within predetermined ranges and for providinglevel difference outputs indicative thereof; and control means,responsive to said seocnd extreme value detecting means and said leveldetermining means, for providing the conditional command to saidreference absolute extreme value point locating means to detect the nextextreme value point when the occurrence and level difference outputsindicate absence of at least one of occurrence and level differenceswithin the predetermined ranges and for providing an output indicativeof the existence of a Korotkoff sound waveform when the occurrence andlevel difference outputs indicate both occurrence and level differenceswithin the predetermined ranges.
 2. An apparatus according to claim 1,wherein said blood vessel information detecting means includes settingmeans for setting a threshold value for detection of the vibration ofthe blood vessel in accordance with the magnitude of the representationof a Korotkoff sound recognized immediately before.
 3. An apparatusaccording to claim 1, wherein said storage means periodically stores aportion of the signal waveform using a predetermined time period.
 4. Ablood pressure measurement method, comprising the steps of:(a) detectingvibration of a blood vessel to produce a signal waveform; (b) storingthe signal waveform produced in step (a) together with time informationassociated therewith; (c) operating on the signal waveform stored instep (b) in response to a conditional command to locate a referenceabsolute extreme value point of the signal waveform, the referenceextreme value point being used as a base point for time durationmeasurement, step (c) comprising the steps of:(ci) detecting one offirst and next extreme value points in the signal waveform stored instep (b); (cii) determining polarity of the one of the first and nextextreme value points and inverting a local portion of the signalwaveform when the polarity is negative; and (ciii) providing thereference absolute extreme value point representative of the one of thefirst and next extreme value points detected in step (ci); (d) findingrelative extreme value points with respect to the reference extremevalue point and the time information to validate existence of aKorotkoff waveform, comprising the steps of:(di) receiving the localportion of the signal waveform; (dii) detecting the relative extremevalue points in the local waveform portion in predetermined timeintervals referenced to the reference absolute extreme value point inthe signal waveform stored in step (b) and providing an occurrenceoutput indicative thereof; and (diii) determining whether leveldifferences between adjacent ones of the reference absolute and relativeextreme value points fall within predetermined ranges and providinglevel difference outputs indicative thereof; (e) providing theconditional command for step (c) to detect the next extreme value pointwhen the occurrence and level difference outputs indicate absence of atleast one of occurrence and level differences within the predeterminedranges; and (f) providing an output indicative of the existence of aKorotkoff sound waveform when the occurrence and level differenceoutputs indicate both occurrence and level differences within thepredetermined ranges.